Mechanical, electronic and optical properties of a novel B2P6 monolayer: ultrahigh carrier mobility and strong optical absorption

Mechanical and thermoelectric properties of ZrX2 and HfX2 (X = S and Se) from Van der Waals density-functional theory

The structural, mechanical, and thermoelectric characteristics of layered transition metal dichalcogenides MX2 (M = Zr, Hf; X = S, Se) have been studied using density functional theory along with van der Waals correction. The exchange-correlation functional, enhanced with corrections for van der Waals interactions, has been evaluated for the hexagonal bulk structures of these materials. The analysis of elastic properties reveals that these compounds exhibit brittleness at zero pressure and conform to Born's criteria for mechanical stability. Examination of elastic constants and moduli suggests that the compounds possess reasonable machinability, moderate hardness, and anisotropy in terms of sound velocity. Transport properties, including the Seebeck coefficient, electrical conductivity, thermal conductivity, and power factor, have been computed using the semi-classical Boltzmann theory implemented in the BoltzTraP code. All investigated compounds exhibit excellent thermoelectric performance at high temperatures. This result suggests that our compounds are highly promising candidate for practical utilization in the thermoelectric scope.

Novel BiOI/LaOXI〈IX〉 heterojunction with enhanced visible-light driven photocatalytic performance: unveiling the mechanism of interlayer electron transition

Improving visible light absorption plays an important role in the utilization of solar power for photocatalysis. Using first-principles calculations within the HSE06 functional, we propose that the semiconductor heterojunction BiOI/LaOXI〈IX〉 extends the optical absorption to the near-infrared range, boosts the absorption coefficient from 1.28 × 105 cm-1 to above 2.20 × 105 cm-1 in the visible light range, and increases the conversion efficiency of solar power up to 9.48%. The enhanced optical absorption derives from the significant interlayer transition and excitonic effect which benefit from polarized LaOXI with a flat band in the highest valence band (VB). In BiOI/LaOClI〈ICl 〉, the electrostatic potential difference (ΔΦ) modifies the band edge positions to meet the requirements for photocatalytic overall water splitting, while the polarized electric field (Ep) accelerates the separation of photogenerated carriers and regulates the overpotentials of photogenerated carriers following a direct Z-scheme strategy. In addition, BiOI/LaOXI〈IX〉 is dynamically and thermodynamically stable. Furthermore, only a low external potential is needed to drive the redox reaction. Our theoretical results suggest that BiOI/LaOXI〈IX〉 could be a potential photocatalyst for overall water splitting with enhanced visible light absorption.

Stacking engineering induced Z-scheme MoSSe/WSSe heterostructure for photocatalytic water splitting

Stacking engineering is a popular method to tune the performance of two-dimensional materials for advanced applications. In this work, Jansu MoSSe and WSSe monolayers are constructed as a van der Waals (vdWs) heterostructure by different stacking configurations. Using first-principle calculations, all the relaxed stacking configurations of the MoSSe/WSSe heterostructure present semiconductor properties while the direct type-II band structure can be obtained. Importantly, the Z-scheme charge transfer mode also can be addressed by band alignment, which shows the MoSSe/WSSe heterostructure is an efficient potential photocatalyst for water splitting. In addition, the built-in electric field of the MoSSe/WSSe vdWs heterostructure can be enhanced by the S-Se interface due to further asymmetric structures, which also results in considerable charge transfer comparing with the MoSSe/WSSe vdWs heterostructure built by the S-S interface. Furthermore, the excellent optical performances of the MoSSe/WSSe heterostructure with different stacking configurations are obtained. Our results provide a theoretical guidance for the design and control of the two-dimensional heterostructure as photocatalysts through structural stacking.

A graphene/Janus B2P6 heterostructure with a controllable Schottky barrier via interlayer distance and electric field

Lowering the Schottky barrier at the metal-semiconductor interface remains a stern challenge in the field of field-effect transistors. Herein, an in-depth investigation was conducted to explore the formation mechanism of the Schottky barrier via interlayer distance and external electric field, utilizing the first-principles approach. Attributed to the vertical asymmetric structure of B2P6, ohmic contact forms at the interface of a graphene/B2P6(001) heterostructure, and an n-type Schottky contact with a Schottky barrier of 0.51 eV forms at the interface of a graphene/B2P6(001̄) heterostructure. Furthermore, the Schottky barrier height and the contact type can be changed by adjusting the interlayer spacing or applying an electric field along the Z direction. A high carrier concentration of 4.65 × 1013 cm-2 is obtained in the graphene/B2P6(001) heterostructure when an external electric field of 0.05 V Å-1 is applied. Verifiably, alterations in the energy band structure are attributed to the redistribution of charges at the interface. The new findings indicate that GR/B2P6 heterostructures are a key candidate for next-generation Schottky field-effect transistor development.

Ultraflexible two-dimensional Janus heterostructure superlattice: a novel intrinsic wrinkled structure

The recently reported two-dimensional Janus transition metal dichalcogenide materials present promising applications such as in transistors, photocatalysts, and thermoelectric nanodevices. In this work, using molecular dynamics simulations, the self-assembled in-plane MoSSe/WSSe heterostructure superlattice is predicted with a natural sinusoidal structure constructed by an asymmetric interface. Such a sinusoidal structure shows extraordinary mechanical behavior where the fracture strain can be enhanced up to 4.7 times than that of the symmetrical interface. Besides, the deformational structure of all these MoSSe/WSSe heterostructure superlattice are in accordance with the Fourier function curve; the fracture strength and fracture strain also demonstrate pronounced size dependence. Our investigations proposed an ultrastretchable assembled heterostructure superlattice and provided a desirable strategy to tune the mechanical properties of such an in-plane two-dimensional heterostructure.

Enhanced Photocatalytic Activity of Two-Dimensional Polar Monolayer SiTe for Water-Splitting via Strain Engineering

A two-dimensional (2D) polar monolayer with a polarization electric field can be used as a potential photocatalyst. In this work, first principle calculations were used to investigate the stability and photocatalytic properties of 2D polar monolayer SiTe as a potential promising catalyst in water-splitting. Our results show that the 2D polar monolayer SiTe possesses an indirect band gap of 2.41 eV, a polarization electric field from the (001) surface to the (001¯) surface, a wide absorption region, and a suitable band alignment for photocatalytic water-splitting. We also discovered that the photocatalytic activity of 2D polar monolayer SiTe could be effectively tuned through strain engineering. Additionally, strain engineering, particularly compressive strain in the range from −1% to −3%, can enhance the photocatalytic activity of 2D polar monolayer SiTe. Overall, our findings suggest that 2D polar monolayer SiTe has the potential to be a promising catalyst for photocatalytic water-splitting using visible light.

Two-dimensional MX2Y4 systems: ultrahigh carrier transport and excellent hydrogen evolution reaction performances

Very recently, two-dimensional MoSi₂N₄ has been synthetized (Y.-L. Hong, Z. Liu, L. Wang, T. Zhou, W. Ma, C. Xu, S. Feng, L. Chen, M.-L. Chen and D.-M. Sun, Chemical vapor deposition of layered two-dimensional MoSi₂N₄ materials, Science, 2020, 369, 670-674.). In this work, we systematically explore the mechanical, electronic, and catalytic properties of the MX₂Y₄ (M = Cr, Hf, Mo, Ti, W, Zr; X = Si, Ge; Y = N, P, As) monolayers by first-principles calculations. These observed monolayers exhibit an isotropic Young's moduli of 165-514 N m^-1 and a Poisson's ratio of 0.26-0.33. The calculated band structures indicate that their bandgaps are in the range of 0.49-2.05 eV at the HSE06 level. In particular, a high electron mobility of about 1.04 × 10⁴ cm² V^-1 s^-1 is observed in TiSi₂N₄ monolayers, which shows potential for high-speed electronic devices. MX₂Y₄ monolayers also reveal decent performances in the hydrogen evolution reaction. More importantly, the Gibbs free energy change of the TiSi₂N₄ (ZrSi₂N₄) monolayer is as small as 0.078 eV (-0.035 eV), even being comparable with that of Pt (-0.09 eV). This investigation suggests that the MoSi₂N₄ family monolayers have potential advanced applications such as photocatalytic, electrocatalytic, and photovoltaic devices.

Investigation of adsorption behaviors, and electronic and magnetic properties for small gas molecules adsorbed on Pt-doped arsenene by density functional calculations

Based on first-principles calculations, we compared the adsorption behaviors, electronic and magnetic properties of gas molecules (CO, NO, NO2, O2, NH3, H2O, H2 and N2) adsorbed on Pt-embedded arsenene and pristine arsenene. Our calculations show the interactions between molecules and arsenene can be enhanced by substitution of a Pt atom, suggesting the potential application of Pt-doped arsenene in gas sensing, especially for detecting NO2 gas due to the largest adsorption energy and charge transfer between NH3 and Pt-doped arsenene. Among all the molecules considered, CO, NO, NO2, O2 and NH3 molecules chemisorb on Pt-doped arsenene (these molecules physisorb on pristine arsenene) forming covalent Pt-C, Pt-N and Pt-O bonds, thus resulting in the elongation of C-O, N-O, O-O, N-H bonds in molecules. The magnetic moments of arsenene adsorbed with O2, NO, and NO2 decrease or diminish after Pt doping. Such variation on magnetism before and after Pt doping is ascribed to significant charge transfer and strong hybridization between gas molecules and the underlying Pt atoms, indicating the magnetic properties of arsenene can be tuned by molecular adsorption and Pt doping. These findings suggest Pt-doped arsenene has potential applications in spintronic devices, catalysts and gas sensors.

Controllable Schottky barriers and contact types of BN intercalation layers in graphene/MoSi2As4 vdW heterostructures via applying an external electrical field

Graphene-based van der Waals (vdW) heterostructures have opened unprecedented opportunities for various device applications due to their rich functionalities and novel physical properties. Motivated by the successful synthesis of a MoSi₂N₄ monolayer (Science, 2020, 369, 670), in this work by means of first-principles calculations we construct and investigate the interfacial electronic properties of the graphene/MoSi₂As₄ vdW heterostructure, which is expected to be energetically favorable and stable. Our results show that the graphene/MoSi₂As₄ heterostructure forms an n-type Schottky contact with a low barrier of 0.12 eV, which is sensitive to the external electric field and the transformation from an n-type Schottky contact to a p-type one can be achieved at 0.2 V Å^-1. The small effective masses and strong optical absorption intensity indicate that the graphene/MoSi₂As₄ heterostructure will have a high carrier mobility and can be applied to high-speed FET. Importantly, we also show that the opening band gap can be achieved in the graphene/BN/MoSi₂As₄ heterostructure and the type-I band alignment can transform into type-II under an external electric field of -0.2 V Å^-1. These findings demonstrate that the graphene/MoSi₂As₄ heterostructure can be considered as a promising candidate for high-efficiency Schottky nanodevices.

The First-Principles Study of External Strain Tuning the Electronic and Optical Properties of the 2D MoTe2/PtS2 van der Waals Heterostructure

Two-dimensional van der Waals (vdW) heterostructures reveal novel properties due to their unique interface, which have attracted extensive focus. In this work, the first-principles methods are explored to investigate the electronic and the optical abilities of the heterostructure constructed by monolayered MoTe₂ and PtS₂. Then, the external biaxial strain is employed on the MoTe₂/PtS₂ heterostructure, which can persist in the intrinsic type-II band structure and decrease the bandgap. In particular, the MoTe₂/PtS₂ vdW heterostructure exhibits a suitable band edge energy for the redox reaction for water splitting at pH 0, while it is also desirable for that at pH 7 under decent compressive stress. More importantly, the MoTe₂/PtS₂ vdW heterostructure shows a classy solar-to-hydrogen efficiency, and the light absorption properties can further be enhanced by the strain. Our results showed an effective theoretical strategy to tune the electronic and optical performances of the 2D heterostructure, which can be used in energy conversion such as the automotive battery system.

Effects of 3d transition metal impurities and vacancy defects on electronic and magnetic properties of pentagonal Pd2S4: competition between exchange splitting and crystal fields

In this paper, we first investigate the electronic properties of the two-dimensional structure of dichalcogenide Pd₂S₄. These properties strongly depend on the crystal field splitting which can change by atomic vacancies (S and Pd vacancies). The main purpose of the present paper is to create remarkable magnetic properties in the system by adding 3d transition metal atoms where the presence of Mn, Cr, and Fe creates the exchange interaction in the system as well as change in the crystal field. The created magnetic properties strongly depend on the competition between exchange interaction and crystal field to separate the levels of d orbitals. In addition, the presence of the transition metals in the structures with S and Pd vacancy has been investigated carefully. The calculations demonstrate that we can achieve an extensive range of magnetic moment up to 3.131 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mu }_{B}$$\end{document}μB. The maximum one is obtained in the presence of Mn and absence of sulfur while some of the doped structures does not have magnetic moment. Our results show that Pd vacancy in the presence of Cr, Mn and Fe metals increases the magnetic property of the Pd₂S₄ structure. The extensiveness and variety of the obtained properties can be used for different magnetic and non-magnetic applications.

Tunable electronic and magnetic properties of Cr2Ge2Te6monolayer by organic molecular adsorption

Recently, two-dimensional materials are widely concerned because of their novel physical properties. Cr₂Ge₂Te₆(CGT) has been studied extensively due to its intrinsic ferromagnetism and ferromagnetic order. In this investigation, the electronic and magnetic performances of organic molecules (TCNE, TCNQ and TTF) adsorbed on CGT monolayer were studied based on the first-principles calculations systematically. The results demonstrate that the CGT presents pronounced tunable electronic and magnetic properties by the adsorption of these macromolecules. Furthermore, the Curie temperature of CGT monolayer can be enhanced significantly by the TTF adsorption. This work can provide a magnetic regulation method for CGT and explore the promising applications of the CGT for spin devices.

Effects of adatom species on the structure, stability, and work function of adatom-α-borophene nanocomposites

Work function-tunable borophene-based electrode materials are of significant importance because they promote efficient carrier extraction/injection, thereby enabling electronic devices to achieve maximum energy conversion efficiency. Accordingly, determining the work function of adatom-borophene nanocomposites within a series wherein the adatom is systematically changed will facilitate the design of such materials. In this study, we theoretically determined that the M-B bond length, binding energy, electron transfer between adatoms and BBP, and work function (ϕ) are linearly dependent on the ionization potential (IP) and electronegativity for thermodynamically and kinetically stable adatom-α-borophene (M/BBP) systems involving a series of alkali (earth) metal/BBP (M = Li-Cs; Be-Ba) and halogen/BBP (M = F-I), respectively. However, the binding energies of Li/BBP and Be/BBP deviate from these dependencies owing to their super small adatoms and the resulting significantly enhanced effective M-B bonding areas. By interpreting the electron transfer picture among the different parts of M/BBP, we confirmed that metallic M/BBP possesses ionic sp-p and dsp-p M-B bonds in alkali (earth) metal/BBP but covalent-featured ionic p-p interactions in halogen/BBP. In particular, the direct proportionality between IP and ϕ for alkali (earth) metal/BBP originates from the synergistic effect of charge rearrangement and the increased induced dipole moment; however, the inverse proportionality between electronegativity and ϕ for halogen/BBP arises from the adsorption induced charge redistribution. Our results provide guidance for experimental efforts toward the realization of work function-tunable borophene-based electrodes as well as insight into the bonding rules between various adatoms and α-borophene.

First-Principles Calculations of Two-Dimensional CdO/HfS2 Van der Waals Heterostructure: Direct Z-Scheme Photocatalytic Water Splitting

Using two-dimensional (2D) heterostructure as photocatalyst for water splitting is a popular strategy for the generation of hydrogen. In this investigation, the first-principles calculations are explored to address the electronic performances of the 2D CdO/HfS₂ heterostructure formed by van der Waals (vdW) forces. The CdO/HfS₂ vdW heterostructure has a 1.19 eV indirect bandgap with type-II band alignment. Importantly, the CdO/HfS₂ vdW heterostructure possesses an intrinsic Z-scheme photocatalytic characteristic for water splitting by obtaining decent band edge positions. CdO donates 0.017 electrons to the HfS₂ layer in the heterostructure, inducing a potential drop to further separate the photogenerated electrons and holes across the interface. The CdO/HfS₂ vdW heterostructure also has excellent optical absorption capacity, showing a promising role as a photocatalyst to decompose the water.

Effect of the sulfur termination on the properties of Hf2CO2 MXene

A computational study was carried out to investigate the effect of surface termination on Janus Hf₂COS MXene by substituting partly the O-terminated layer with S atoms. Our predictions confirm that this chemical strategy allows one to tailor the band gap of MXenes. Indeed, the semiconducting character of Hf₂CO₂ MXene decreases by the exchange of O by S atoms. From a structural point of view, dynamical, mechanical, and thermal analysis confirm the thermodynamic stability of the Janus Hf₂COS MXene, which shows metallic character. Furthermore, topological chemical analysis indicates an ionic nature of Hf₂CO₂ MXene that tends to be reduced by increasing the concentration of S atoms, promoting a covalent character. Shortly, the present study illustrates how the properties of MXenes can be tailored by functionalizing them with different chemical terminations.

Stacking-Mediated Type-I/Type-II Transition in Two-Dimensional MoTe2/PtS2 Heterostructure: A First-Principles Simulation

Recently, a two-dimensional (2D) heterostructure has been widely investigated as a photocatalyst to decompose water using the extraordinary type-II band structure. In this work, the MoTe2/PtS2 van der Waals heterostructure (vdWH) is constructed with different stacking structures. Based on density functional calculations, the stacking-dependent electronic characteristic is explored, so that the MoTe2/PtS2 vdWH possesses type-I and type-II band structures for the light-emitting device and photocatalyst, respectively, with decent stacking configurations. The band alignment of the MoTe2/PtS2 vdWH is also addressed to obtain suitable band edge positions for water-splitting at pH 0. Furthermore, the potential drop is investigated, resulting from charge transfer between the MoTe2 and PtS2, which is another critical promotion to prevent the recombination of the photogenerated charges. Additionally, the MoTe2/PtS2 vdWH also demonstrates a novel and excellent optical absorption capacity in the visible wavelength range. Our work suggests a theoretical guide to designing and tuning the 2D heterostructure using photocatalytic and photovoltaic devices.

Dissolution of Portlandite in Pure Water: Part 2 Atomistic Kinetic Monte Carlo (KMC) Approach

Portlandite, as a most soluble cement hydration reaction product, affects mechanical and durability properties of cementitious materials. In the present work, an atomistic kinetic Monte Carlo (KMC) upscaling approach is implemented in MATLAB code in order to investigate the dissolution time and morphology changes of a hexagonal platelet portlandite crystal. First, the atomistic rate constants of individual Ca dissolution events are computed by a transition state theory equation based on inputs of the computed activation energies (ΔG*) obtained through the metadynamics computational method (Part 1 of paper). Four different facets (100 or 1¯00, 010 or 01¯0, 1¯10 or 11¯0, and 001 or 001¯) are considered, resulting in a total of 16 different atomistic event scenarios. Results of the upscaled KMC simulations demonstrate that dissolution process initially takes place from edges, sides, and facets of 010 or 01¯0 of the crystal morphology. The steady-state dissolution rate for the most reactive facets (010 or 01¯0) was computed to be 1.0443 mol/(s cm2); however, 0.0032 mol/(s cm2) for 1¯10 or 11¯0, 2.672 × 10-7 mol/(s cm2) for 001 or 001¯, and 0.31 × 10-16 mol/(s cm2) for 100 or 1¯00 were represented in a decreasing order for less reactive facets. Obtained upscaled dissolution rates between each facet resulted in a huge (16 orders of magnitude) difference, reflecting the importance of crystallographic orientation of the exposed facets.

Dissolution of Portlandite in Pure Water: Part 1 Molecular Dynamics (MD) Approach

The current contribution proposes a multi-scale bridging modeling approach for the dissolution of crystals to connect the atomistic scale to the (sub-) micro-scale. This is demonstrated in the example of dissolution of portlandite, as a relatively simple benchmarking example for cementitious materials. Moreover, dissolution kinetics is also important for other industrial processes, e.g., acid gas absorption and pH control. In this work, the biased molecular dynamics (metadynamics) coupled with reactive force field is employed to calculate the reaction path as a free energy surface of calcium dissolution at 298 K in water from the different crystal facets of portlandite. It is also explained why the reactivity of the (010), (100), and (11¯0) crystal facet is higher compared to the (001) facet. In addition, the influence of neighboring Ca crystal sites arrangements on the atomistic dissolution rates is explained as necessary scenarios for the upscaling. The calculated rate constants of all atomistic reaction scenarios provided an input catalog ready to be used in an upscaling kinetic Monte Carlo (KMC) approach.

Two-Dimensional PtS2/MoTe2 van der Waals Heterostructure: An Efficient Potential Photocatalyst for Water Splitting

Recently, the energy shortage has become increasingly prominent, and hydrogen (H2) energy has attracted extensive attention as a clean resource. Two-dimensional (2D) materials show excellent physical and chemical properties, which demonstrates considerable advantages in the application of photocatalysis compared with traditional materials. In this investigation, based on first-principles methods, 2D PtS2 and MoTe2 are selected to combine a heterostructure using van der Waals (vdW) forces, which suggests a type-II band structure to prevent the recombination of the photogenerated charges. Then, the calculated band edge positions reveal the decent ability to develop the redox reaction for water splitting at pH 0. Besides, the potential drop between the PtS2/MoTe2 vdW heterostructure interface also can separate the photogenerated electrons and holes induced by the charge density difference of the PtS2 and MoTe2 layers. Moreover, the fantastic optical performances of the PtS2/MoTe2 vdW heterostructure further explain the promising advanced usage for photocatalytic decomposition of water.

Ab Initio Calculations for the Electronic, Interfacial and Optical Properties of Two-Dimensional AlN/Zr2CO2 Heterostructure

Recently, expanding the applications of two-dimensional (2D) materials by constructing van der Waals (vdW) heterostructures has become very popular. In this work, the structural, electronic and optical absorption performances of the heterostructure based on AlN and Zr2CO2 monolayers are studied by first-principles simulation. It is found that AlN/Zr2CO2 heterostructure is a semiconductor with a band gap of 1.790 eV. In the meanwhile, a type-I band structure is constructed in AlN/Zr2CO2 heterostructure, which can provide a potential application of light emitting devices. The electron transfer between AlN and Zr2CO2 monolayer is calculated as 0.1603 |e| in the heterostructure, and the potential of AlN/Zr2CO2 heterostructure decreased by 0.663 eV from AlN layer to Zr2CO2 layer. Beisdes, the AlN/Zr2CO2 vdW heterostructure possesses excellent light absorption ability of in visible light region. Our research provides a theoretical guidance for the designing of advanced functional heterostructures.